I have had two occasions in the last few days to go back and try to change the condition on a trigger. In each case I had to delete the trigger and create it again. I have since found I don't have to do that, description below.
I created a simple rule with a basic conditional trigger of between two times, see screenshot. If I try to change the condition it shows new condition in the drop down but when I select it acts the same as if I selected the original between times trigger.
I thought that maybe if I create a new condition in the condition manage perhaps I could use that. I tried it and that does work. So I can change the trigger to the Private boolean condition, created later in the condition manager. You cannot create a new condition when editing a existing conditional trigger, however. Not a big deal just thought I would mention it.